1. Field of the Invention
This invention relates to a video camera zoom lens used in a video movie, a monitoring video camera or the like.
2. Description of the Prior Art
A general construction of a focusing mechanism in a conventional video camera zoom lens of this type is shown in FIGS. 1 and 2.
In the conventional construction shown in FIGS. 1 and 2, a male helicoid 1 is fixedly mounted on a lens mount 2, and has a male helicoid screw (thread) portion 1a formed on an outer peripheral surface of a front portion thereof A female helicoid 3 supports a focusing lens 4 within a front portion thereof, and has a female helicoid screw (thread) portion 3a formed on an inner peripheral surface of a rear portion thereof The female helicoid screw portion 3a of the female helicoid 3 is threaded on the male helicoid screw portion 1a of the male helicoid 1 so that the female helicoid 3 can be moved back and forth so as to effect the focus adjustment.
Therefore, in such a threaded condition, the female helicoid 3, because of its dead-weight a, is decentered downward with respect to the male helicoid 1 by an amount corresponding to a gap between the threaded helicoid screw portions 1a and 3a. Thus, the amount (-b) of decentering between the male and female helicoids 1 and 3 is produced, and a supporting point c for this threaded connection lies at the upper section of the male helicoid 1 threadedly engaged with the female helicoid 3.
In the above conventional focusing mechanism in which the female helicoid screw portion 3a is threaded on the male helicoid screw portion 1a of the male helicoid 1, when the focus adjustment is to be effected, the lower portion of the female helicoid 3 is angularly moved (or screwed) manually as at d or by a focus motor e, so that the focusing lens 4 is moved back and forth by this angular movement (screwing motion).
However, in the conventional focusing mechanism of the above construction for the video camera zoom lens, when the focus adjustment is to be effected manually, the female helicoid 3 is first lifted by hand from the lower side thereof and is angularly moved, because the thread supporting point c supporting the female helicoid 3 lies at the upper side, with the decentering amount (-b) produced between the male helicoid 1 and the female helicoid 3. Then, after the focus adjustment is finished, the hand is released from the female helicoid 3. Therefore, during this operation, the female helicoid 3 and hence the focusing lens 4 are moved upward and downward, and as a result the picked-up image shakes upward and downward (that is, undergoes pitching). In the case of the electrical operation for the focus adjustment, similarly, the thread supporting point c supporting the female helicoid 3 lies at the upper side, with the decentering amount (-b) produced between the male helicoid 1 and the female helicoid 3), and the torque operating point by the focus motor e is usually disposed at the lower side, and therefore the focusing lens 4 is swung right and left like a pendulum during the time from the start of the rotation of the motor e to the end of the rotation of this motor. As a result, the picked-up image shakes right and left (that is, undergoes rolling). Thus, the above undesirable problems are encountered in manual operation and electrical operation.
Further, when the supporting point c for the threaded connection is disposed at the upper side with respect to the male helicoid 1, the focusing lens 4 is swung right and left like a pendulum from the start of the rotation of the motor e to the end of the rotation of this motor even if an actuating point of the torque provided by the focus motor e is disposed at any position. Thus, undesirable problems are encountered in the above operations as well.